The present invention relates to an aerosol generator that will deliver a high volume of polystyrene latex (PSL) spheres of a uniform particle size to form an aerosol spray, under precise control.
Fibrous and membrane filters are widely used in industrial gas cleaning systems to remove suspended particulate matter in air or other gases to avoid product contamination by the suspended particles. In critical applications, a high efficiency filter is needed. Filters of the highest efficiency are usually referred to as High Efficiency Particulate Air (HEPA) filters. The traditional HEPA filter is defined as a filter with efficiencies of 99.97% based on tests made with di-octyl phthalate (DOP) aerosols of 0.3 .mu.m diameter. In recent years, filters of efficiencies higher than 99.97% have been developed. These filters are referred to as Ultra-Low Penetration Air (ULPA) filters. ULPA filters usually have efficiencies that are equal to or higher than 99.999%. Filters with 6-nine efficiencies (99.9999%), or 7-nine efficiencies (99.99999%) are also commercially available. These filters are widely used in the semiconductor and microelectronics industries for air filtration in cleanrooms. Other significant users of HEPA and ULPA filters include the nuclear industry, the pharmaceutical and biotechnology industries, hospital operating rooms, and other facilities in the health care industry, among others.
Before a HEPA or an ULPA filter can be accepted for use in a cleanroom, it must be tested to make sure that its efficiency is equal to or higher than that specified by the manufacturer. Although di-octyl phthalate (DOP) is widely used as the aerosol material by the nuclear and pharmaceutical industries for testing high efficiency filters, its use in the semiconductor and microelectronics industries is very limited. DOP is an oily substance with a finite vapor pressure. Following testing by DOP, the filter is contaminated by the collected oil droplets, which will evaporate over long periods to cause organic contamination of the cleanroom air. For this reason, DOP is generally not acceptable in the semiconductor and microelectronics industries.
A method that is becoming more popular for testing HEPA and ULPA filters in the semiconductor and microelectronics industries is the polystyrene latex (PLS) aerosol test. In the PSL test, an aqueous suspension of the PSL spheres of a uniform particle size is prepared, which is then atomized to form an aerosol spray. By evaporating the water from the atomized droplets, a uniform suspension of PSL spheres in air is obtained. These PSL spheres are then used as the aerosol challenge for testing the filter. The filter efficiency can be measured by counting the particles upstream and downstream of the filter by a laser particle counter, or other light scattering particle counters. The size of PSL spheres to be used for HEPA and ULPA filter testing in the semiconductor and microelectronics industry is usually specified by the buyer of the filters. PSL sphere sizes of 0.15 .mu.m and 0.2 .mu.m are quite common. Filter testing by DOP and other aerosol sources are specified in a number test standards.
Two methods have been used to atomize the PSL suspension so far by those involved in filter testing for the cleanroom industry. In one method, the PSL suspension is atomized by an ultrasonic nebulizer. The nebulizer output is then fed to the filter test system to form an aerosol challenge for the filter. In another method, a compressed air nebulizer of the type used for medical applications is used for atomization. In both cases, because the output of the nebulizer is quite small--typically from 0.1 cubic foot per minute (cfm), to approximately 1 cfm--and the challenge air flow for the filter is quite large (typically 800 cfm for a 2 foot.times.4 foot filter) it is necessary to use many nebulizers in parallel in order to produce an adequate aerosol concentration in the challenge air flow for filter testing. Sometimes as many as twenty nebulizers must be used in parallel to generate an aerosol at a high enough flow rate to produce a sufficient particle concentration for filter testing.
A second problem with the current method of generating PSL spheres for aerosol testing is that the nebulizers that are currently available produce relatively coarse droplets that result in high consumption rate for the PSL suspensions. Since uniform sized PSL suspensions are very expensive, the cost of PSL consumed for these tests is also very high. It is not uncommon for a manufacturer to spend as much as $1,000 per day for PSL spheres in testing filters.